Polyimides, e.g. bismaleimide functional compounds, are useful monomers that have found wide applications in composite resins, adhesives, and moulding compounds. Such polyimides are known to be capable of polymerizing to yield polymerization- and polyaddition products possessing high glass transition temperature, high modulus, and good heat resistance properties.
However, it is commonly known that members of an important class of bismaleimides, i.e. aromatic mono-nuclear or aromatic polynuclear N,N′-bismaleimides possess poor handling properties, since they have high melting points and only poor solubility in common solvents. Due to these properties the application of curable mixtures containing such N,N′-bismaleimides is often associated with processing difficulties. In addition, in the past it has been found that cured products based on curable mixtures comprising such N,N′-bismaleimides are brittle, and show high moisture absorption, which results in poor hot/wet performance. Accordingly, many of the conventional curable mixtures based on such N,N′-bismaleimides are severely limited in their applications.
U.S. Pat. No. 4,351,932, for example, describes mixtures comprising N,N′-bismaleimides or mixtures of N,N′-bismaleimides and divinylbenzene as comonomer for the application as prepreg resins. These resins have only poor processing properties because of their short pot life and short gel time. Prepregs made therefrom have a short out time and poor tack and drape. Furthermore, the cured products are brittle and show high water uptake.
EP 0 469 684 A1 discloses mixtures comprising N,N′-bismaleimides or mixtures of N,N′-bismaleimides and alkenylphenoxyalkanes as comonomers. Although these curable mixtures possess relatively low viscosity in the molten state, they suffer from crystallization instability, which makes them almost unusable for the manufacture of fibre-reinforced composites via hot melt techniques, such as filament winding and resin transfer moulding. In addition, their solubility in common solvents is poor, which limits their applicability for solvent/solution prepregging.
US 2008/0075965 A1 discloses adhesive formulations comprising only one maleimide and an aromatic diene, or an aromatic monoene. The favoured compositions of this application are based on 1,3-diisopropenylbenzene and oligomers thereof, or on compounds that carry isopropenyl functional groups combined with limonene-bismaleimide as the favoured maleimide. Such compositions are extremely fast-curing and are, therefore, unsuitable for the manufacture of fibre-reinforced composites via filament winding, resin transfer moulding and the like. The mixtures suffer from viscosity instability due to rapid resin advancement.
US 2012/0049106 A1 discloses amorphous low-melting bismaleimide mixtures, which are synthesized by employing a mixture of maleic anhydride and alkenyl-substituted succinic anhydride. However, these mixtures, when cured in the presence of co-monomers, provide cured products that suffer from low glass transition temperature and reduced long-term thermal stability due to the long aliphatic side chains of the succinic anhydride precursor.
Therefore, there is a need for curable mixtures based on polyimides, which are tough and heat-resistant after cure, and which provide improved processing properties as hot melts and/or as solutions to fiber-reinforced composites.
As many of the processing difficulties of curable mixtures containing aromatic mono-nuclear or aromatic polynuclear N,N′-bismaleimides as well as many of the limitations for their application are associated with high melting points and poor solubility in common solvents of the bismaleimides employed, attempts have been undertaken in the prior art to find bismaleimides or mixtures thereof that avoid these drawbacks. EP 0469684A1, for example, discloses that eutectic mixtures of bismaleimides can be used in order to lower the melting point. In particular, EP 0469684A1 (examples 5-11), discloses that the eutectic mixture of 4,4′-bismaleimidodiphenylmethane and 2,4-bismaleimidotoluene may be employed in combination with bis(alkenylphenoxy)alkane as a comonomer, alone or in combination with other comonomers, for tough cured products. However, as indicated above, although these curable mixtures possess relatively low viscosity in the molten state, they suffer from crystallization instability, which makes them almost unusable for the manufacture of fibre-reinforced composites via hot melt techniques, such as filament winding and resin transfer moulding. In addition, their solubility in common solvents is poor, which limits their applicability for solvent/solution prepregging.
It is, therefore, an object of the present invention to provide curable mixtures that are low melting, possess low viscosity at the lowest possible temperature, that are, further, stable at the processing temperature in terms of no or only slight (low viscosity) advancement during processing, at least for a time sufficient to manufacture a part, and which are, in addition, stable in terms of no crystallization of resin components throughout the manufacturing process.
It is a further object of the present invention to provide curable mixtures and curable prepolymers, which are processable to fibre-reinforced composites with the use of techniques, requiring stable, low-viscosity melt resins, such as filament winding (FW), hot melt prepregging, resin transfer moulding (RTM), and resin infusion moulding (RIM).
It is a further object of the present invention to provide low-melting and low-viscosity prepolymers, which are stable with respect to crystallization and resin advancement in the molten state.
It is a further object of the present invention to provide curable mixtures and curable prepolymers, which are soluble in low-boiling solvents comprising 1,3-dioxolane, and which form stable solutions with respect to resin crystallization and advancement.
The problem underlying the present invention, thus, resides in providing curable mixtures exhibiting above-mentioned desired characteristics. It has been found that this problem is solved by the curable mixtures defined below.